DSC range / response

[webcraft]

On a recent passage we received a DSC distress message (undesignated) and I automatically pressed the acknowledge button.

I then checked the position of the craft in distress, and it was over 200 miles away. At five knots we were obviously not going to be of much help.

Who gets the acknowledgement, the sending craft? If so I was just cluttering up the airwaves and possibly giving false hope. Presumably it is better to not acknowledge the message until you have checked the position - or am I missing something?

- W

 

[AlistairC]

I didnt think that the class of set that we have could send acknowledgements?

 

[fireball]

With class D you've just silenced the alarm ... no transmissions will be made by your set. The originating set will send out another signal if nobody with a ?class A? set acknowledges them.

 

[SvenglishTommy]

I was also under the impression that you cannot acknowledge a DSC distress call with a class D set. Also, I do not see how you could pick up a signal 200 miles away. A DSC call may manage to get a through where a voice call would be garbled, but it still follows the principle of line of sight. The boat in distress must have had very old position data typed in.

Did you hear any follow-up on 16?

 

[webcraft]

The relevant instructions in the manual for the radio are reproduced verbatim below:

----------------------------------------------------
Read message or Exit

Press Read - the READ mode screen is displayed and the message details are now shown. To acknowledge the message press SELECT

You now have a choice. Transmit acknowledge?

Press YES (default) or NO

(Note - normal practice is to acknowledge a received DSC message, especially Distres or Urgency messages)

Press SELECT to transmit a DSC acknowledgement. A DSC Acknowledge message is transmitted and the display briefly shows ACKNOWLEDGEMENT SENT and the MMSI of the called station. The radio now switches to the caller proposed channel ready for comunication.
----------------------------------------------------

Seems clear enough to me that the acknowledgement is sent back to the vessel transmitting the distress signal.

The radio is a McMurdo F1 DSC.

- Nick

 

[AHoy2]

The DSC acknowledgment you are refering to is for "individual calls", not priority distress/urgency/safety. Hence the comment about the radio changing to the designated channel.

(Must admit, the reference in your manual seems strange)

 

[mel80]

Regardless of whether you can ackowledge with DSC or not, you should always try to acknowedge by voice in preference. Acknowledging by DSC will stop the station in distress from transmiting his alert, which may prevent other vessels or (more importantly) coast stations from recieving the alert if they didn't get it first time round.

 

[chriscallender]

Well, I would have pressed the acknowledge button (whether it transmits an acknowledgement I'll leave for the DSC experts) and at the very least make every possible attempt to relay the information to the coastguard (assuming I don't hear someone else responding).


There is also the fairly strong possibility that the GPS position in the DSC message is duff (an old one stored in the radio), and in reality they are much closer. So trying to contact them on ch16 voice would probably be even more important than just relaying the DSC alert.

 

[mel80]

[ QUOTE ]
Well, I would have pressed the acknowledge button


[/ QUOTE ]

I guess that's why the class D sets can't acknowledge DSC distress alerts. Even on the very basic RYA VHF course it is made very clear that the order of preference is.

1. Let coastal station acknowledge.
2. Acknowledge by voice of ch16
3. Acknowledge by DSC (impossible for us)

To acknowledge by DSC immediately means that you may prevent stations (that may be in a much better postition to help) from hearing the alert.

 

[AHoy2]

I believe the recommended practice is to listen out for a response from a MRCC or an appropriate vessel (e.g. warship, large commercial vessel) and only contribute if there is any positive action you can take or you are certain that there has been no response. Bear in mind that you may not be able to receive responses if you in range of casualty but out of range of responder.

 

[FAITIRA]

I have talked on VHF with a vessel 600 miles away, believe it or not. Also heard transmissions when in the Irish sea orginating from the North sea. Have been told by "experts" it,s due to skip in unusual weather coditions.

 

[webcraft]

This was an individual distress DSC message which transmitted the mmsi number of the sender and the lat/long.

The message on the radio after I had pressed acknowledge clearly said that the acknowledgement had been sent to the transmitting station.

According to the experts here our radio was lying to me and the manual is also lying. I find this unlilkely so I presume I am hallucinating or in posession of an illegal set?

As regards responding on voice - we had heard a Pan-Pan relay from Las Palmas traffic some hours previously from the same general area - very faint and broken - so we assumed it was the same vessel. There is no point in trying to acknowledge by voice when we can no longer hear the high power Las Palmas radio shore transmissions ourselves. We maintained a listening watch on 16 thereafter and heard nothing else.

I do not know how out of date positions can be - our VHF stops working and beeps incessantly if it has not been updated in the previous two hours. Normally of course it is updated continuously from the GPS, butif for any reason this is not connected then a position has to be input manually to continue using the radio. Perhaps this is not the case with later models?

The only person who in any way answered the question was the person who said no, we shouldn't have acknowledged it because it would block other transmissions. I agree and in any future ocurences will no longer acknowledge an individual DSC distress call by DSC acknowledgement until I have checked the position.

No-one has answered the how or how much question re. DSC transmission range obviously being much greater than voice.

- W

 

[fireball]

I don't know your specific VHF ... but ours cannot acknowledge a distress call - you can press any button you like and it just cancels the alarm on my set.

Don't know about the DSC Range being that much further - it is further than voice comms as it is just a series of on/off signals, but I wouldn't expect to reach more than 25miles ship2ship so you're lucky to receive 200 miles or so....

 

[Twister_Ken]

On range, a couple of points.

Generally VHF can achieve astonishing ranges through a phenomenon called (I think) channelling, or maybe tunneling. I've been in the channel and able to hear every CG station from Brixham to Dover as well as Jersey and assorted French and Belgians. As someone said elsewhere, they have managed to have a conversation at 600 miles range.

Then, as a binary signal on a dedicated channel being received by a gizmo rather than a human ear, weak signals stand a much better chance of being received and interpreted. A Solent CG told me that they regularly hear yacht DSC transmissions from the French side.

 

[SvenglishTommy]

I dont think anyone is suggesting your DSC is lying, or that you were hallucinating. The reason people are not answering your original question is that you are claiming your DSC has the ability to acknowledge DSC distress calls, which it is not supposed to do. You need class A or B DSC equipment to be able to acknowledge DSC distresses. So either have you have a higher class of DSC radio, or the radio is illegally providing you with the functionality to acknowledge DSC distresses.

The issue of your acknowledgment is not about cluttering up the airways, DSC signals take a fraction of a second to transmit, and DSC distress calls are transmitted 5 times on the off-chance there is a clash. The issue is that you may have canceled this guys mayday with no intention of doing anything about it. Once his DSC receives the acknowledgment it will stop transmitting the mayday every 4 minutes.

I am no expert, but I assume there are guidelines for the coast guard and military/commercial ships carrying class A/B DSC equipment which are something along the lines of 'Only acknowledge the distress if you are in a position to do something about it'

 

[morgandlm]

I note that in your biog you say that you do not have a DSC number because you do not have a licence ... do you have a non-standard radio or are you just confused about it's use?
Morgan

 

[Birdseye]

It is possible that the signal was 200 miles away - a fellow radio ham does VHF competitions and regularly contacts stations up to 800 km away, admittedly with 100 watts and a directional area, but still not line of sight. That said, line of sight is all you can RELY on getting.

It could of course be some scumbag who has nicked a DSC radio and is giving false alarms with false positions.

The type of DSC controller you have is incapable of switching off the auto repeat when you acknowledge a mayday call.

 

[starfire]

Tropospheric Ducting.

On one night, I was getting contacts into Germany, France & Belgium from the middle of the UK, using 5 watts, FM on 145Mhz. Not too far removed from marine VHF frequencies.

 

[chrisb]

this may be irrelevent but i was listening to falmouth coastguard whilst in the azores last year. i guess this depends on atmospheric conditions.similarly i was able to talk with falmouthcoastguard whilst 200nm off on the return leg

 

[webcraft]

I don't have the DSC upgrade to my VHF license because I could never find anyone/anywhere to do it in spite of looking for three years. (One company in Oban offered to do it for £120 for a half day, otherwise I had to travel to Glasgow).

Now no doubt we will be arrested on our return to UK waters and the vessel impounded . . . (Should have listened to people who tell you how dangerous it is to put anything in your bio).

I did not think I was confused as to the radio's functions and method of operation - I always assumed that pressing acknowledge merely cancelled the alarm on my set. However, this time it said acknowledgement transmitted, and when I checked the manual it seemed to say that in the event of the acknowledge button being pressed on receipt of a DSC distress call the radio puts out put out a DSC reply saying that the message has been acknowledged. Now I am not so sure. I have looked up various sources on the web and posters here are of course quite right in saying that ships should NOT acknowledge a DSC distress call or relay by DSC, but by voice on ch16 - and then only after waiting a suitable time for coast stations to reply.

It seems very strange that our set should (apparently) offer the facility to do this. The radio was state of the art when I won it in a Tom Cunliffe YM competition. It is still on sale, but it may be that it has features that are not available on standard sets today. Does anyone else have one of these?

(I suppose it is possible that the radio shares some software with class A sets made by the same company in which case the manual and display may simply be wrong and pressing acknowledge does not actually send a transmission.)

The reported position was not far off the very busy port of Las Palmas, and I would imagine that the very efficient Las Palmas radio co-ordinated all necessary rescue efforts. I am just astounded that we received the call from such a distance. Response by voice would have been pointless and I had pressed acknowledge before I thought. Like the other posters here I assumed it would just cancel the signal to my set, - which is all it seems to do with All Ships messages from coast stations - and was a bit amazed when it said 'acknowledgement transmitted to (MMSI no)' I think if it had been sending acknowledgements to coast stations in the past someone would have told me about it.

In response to another poster, it never ocurred to me that my pressing acknowledge would prevent the mayday from being retransmitted. I expected it to merely cancel the transmission to my set. When it appeared to send an acknowledgement my worry was that the distressed vessel would receive the acknowledgement and assume that someone was doing something. I assumed though the acknowledgement would show my MMSI number and position and they will then see that I am not in a position to do anything.

Anyway - it would seem that either:

- my radio is doing something it shouldn't and I will stop using the 'acknowledge' feature for all types of message - although it doesn;t seem to have ever caused any problems with 'All Ships' calls from coastal stations, one of which would surely have chinned me by now ifI had been sending DSC acknowledgements
or

- my radio is saying it is doing something it isn't and pressing acknowledge only does anything at my end and does not send out a DSC message

In any event - in future I will certainly NOT acknowledge any DSC distress messages by pressing acknowledge on the set . . . just in case. I will also query McMurdo on this.

- N